The invention relates to a method for producing metal alloys with a desired concentration of a gas component relevant to their properties, in a vessel which is not entirely gas-tight, as well as a device for implementing this method. The invention relates in particular to a remelting process for producing a steel with a high content of nitrogen which content has to be able to be set precisely.
For quite some time there has been great interest in the use of steels containing nitrogen. Such materials include e.g. non-corroding austenitic alloys which apart from chromium and manganese comprise dissolved nitrogen in quantities of 0.5 to 1.5 mass %. Such steels feature an unusual combination of extreme strength and toughness in addition to exceptional corrosion resistance. Thus e.g. a steel alloy marketed under the trade name of xe2x80x9cP900xe2x80x9d by the applicant, said steel alloy containing 18% chromium, 18% manganese as well as 0.6-1.2 mass % nitrogen, is used for forging cap rings which meet the very exacting technical requirements for incorporation in power generators of large power stations.
New martensitic tool steels are another example of steels containing nitrogen; apart from an increased content of dissolved nitrogen, these steels also comprise hard phases in the form of precipitated nitrides and/or carbonitrides. These materials combine good hardness and wear resistance with extraordinary corrosion resistance. For example an alloy marketed under the trade name of xe2x80x9cCronidurxe2x80x9d by the applicant, produced by the company xe2x80x9cVSG Energie- und Schmiedetechnikxe2x80x9d, said alloy comprising 15% chromium, 1% molybdenum, 0.3% carbon as well as 0.38 W nitrogen has been found to be particularly resistant to corrosion wear in bearings of aircraft engines.
In the applications mentioned, the nitrogen represents the essential element of the alloy which has a deciding influence on the characteristics of the respective material. Even small deviations in concentration can cause a dramatic deterioration in the positive characteristics, e.g. in relation to toughness and the corrosion resistance. For this reason, with such steels precise control of the nitrogen content is of the utmost importance when manufacturing the alloys.
In practical application, steels of the type at issue often have to be melted at increased pressure to be able to achieve the required pressures. Thus the partial pressure of the nitrogen is of particular importance. It causes dissolution of the required quantity of nitrogen in the melt. In addition, in particular in the case of alloys where the solubility of the nitrogen reduces strongly as solidification progresses, the total pressure prevents the formation of gas pores during solidification if said total pressure is above the partial pressure of the nitrogen.
Due to the necessity of melting the steels at issue in a closed pressure vessel, it is not possible to carry out the usual process control involving repeated specimen sampling from the metal bath, and subsequent gas analysis.
In the counterpressure process according to Rashav (Rashev T.-. Big-Size Steel Bath Process for Large Scale Industrial Production of High Nitrogen Steels; Proceedings of 3rd Int. Conference HNS 93, Kiev, Ukraine, 14.-16.9.1993) a steel charge is inductively melted in a pressure vessel and subsequently poured into a permanent mould or a form, while the pressure is maintained. The melt is exposed to increased nitrogen pressure for an extended period so that the desired nitrogen content can find its equilibrium by a nitrogen partial pressure selected for the work temperature according to Sieverts law. However, there is a problem in this connection in that the diffusion coefficient of nitrogen in the metal phase is relatively low compared with other gases. Therefore, in many cases the desired equilibrium only occurs after comparatively long treatment times. The associated long waiting times render the production process uneconomical.
Remelting methods, in particular the electroslag refining process, play an important role in the production of alloys with a high nitrogen content. In such processes, in a first step, an alloy containing less nitrogen is made using the usual means of melt metallurgy, e.g. ladle metallurgy, at normal pressure and is then poured to form an electrode. This electrode provides the starting material for a second process step in which said electrode is remelted at increased pressure. Apart from the increase in the degree of purity which is usually attained during remelting, the remelting process makes it possible to supply nitrogen to the base material of the electrode. In this process, the slag not only serves as a heating conductor but also as a refining reaction partner. In the known adding of nitrogen by alloying, a third function of the slag, which during remelting completely covers the melt, consists of supplying the nitrogen to the melt.
According to the state of the art, adding nitrogen by alloying preferably takes place by adding materials containing nitrogen, said materials being applied onto the slag as a powder mixture or granulate mixture. There is however a problem in that in such an approach, precise adding of nitrogen by alloying according to the state of art, cannot be guaranteed.
To overcome this problem, for example U.S. Pat. No. 4,027,720 proposes the use of a composite electrode in which an alloy rich in nitrogen forms the core while an alloy with little nitrogen forms the sheath. After the remelting process, in this way an ingot of an alloy comprising the desired nitrogen content is to be obtained. Practical application has however shown that this expensive and elaborate process often produces ingots of inhomogeneous composition.
It is thus the object of the invention, starting from the above-mentioned state of the art, to create a method of the type mentioned in the introduction and to create a device which is particularly suited to such a method, which enable precise adding by alloying of an alloying constituent which is gaseous per se, to a steel alloy which is being subjected to overpressure in a pressure vessel.
According to the invention this object is met, starting with a method for producing a metal alloy in which in a vessel at overpressure a certain desired content of an alloying constituent which is gaseous in its normal state, is fed to said metal alloy,
in that the gaseous alloying constituent and an inert gas are added in such quantities to the atmosphere contained in the vessel that a particular initial concentration ratio between the gaseous alloying constituent and the inert gas exists in the atmosphere;
in that the change over time in the total pressure, the change over time in the concentration of the inert gas and in the gaseous alloying constituent in the atmosphere contained in the vessel, as well as the change over time in the mean gas temperature of the atmosphere contained in the vessel, are determined;
in that taking into account the respective total pressure, the respective concentration of the inert gas, the respective concentration of the gaseous alloying constituent, as well as the respective gas temperature, the inert gas mass loss, occurring as a result of lack of tightness of the vessel, of the atmosphere contained in the vessel, is determined;
in that by means of the initial concentration ratio a theoretical mass loss of the gaseous alloying constituent in the atmosphere is determined, said mass loss being the result of lack of tightness;
in that by means of a comparison of the theoretical mass loss with the actual mass loss which is determined taking into account the respective actual concentration of the gaseous component in the atmosphere contained in the vessel, the mass fraction of the gaseous component which has made the transfer to the metal alloy is determined; and
in that taking into account the mass fraction which has made the transfer to the metal alloy and the mass losses of the gaseous alloying constituent which has been lost by lack of tightness, this alloying constituent is admixed to the atmosphere contained in the vessel for such a period of time and in such quantities that after completion of the process, the metal alloy has the desired content of the gaseous alloying constituent.
The method according to the invention makes it possible for the first time to determine on an ongoing basis the materials transfer of a gas component relevant to the quality of a metal alloy to the metal bath during the melting or remelting process in a vessel which is not completely gas-tight. In this way it becomes possible to produce with high precision a material with a desired concentration of the relevant gaseous alloying constituent. The excellent accuracy with which the transfer of the gaseous constituent to the metal alloy can be controlled with application of the method according to the invention, results in an even, homogenous distribution of the alloying constituent absorbed into the metal alloy.
In relation to the preferred field of application of the invention, namely the production of highly nitrogenised steels, for which production with equal preference the pressure-electroslag refining process is used, this means among other things, that an ingot correspondingly produced from the metal alloy, has an even nitrogen concentration profile along its longitudinal axis. If required, the nitrogen, corresponding to the permeability of the slag, can be alloyed with the maximum nitrogen able to be fed in from the atmosphere at the working conditions of the plant. In this way the addition of nitrogen via solid additives containing nitrogen, said addition often being problematic, can be reduced to a minimum or can be done without entirely.
The method according to the invention is in particular suitable for implementation in a vessel which is not completely gas-tight. Due to the size and complexity of such vessels it is almost impossible in practical application, to guarantee constant tightness during operation. At each of the many flanges, valves and connections required for supplying materials to the vessel, there is a danger of gas escaping. Due to the fact that according to the invention the mass fractions of the inert gas and the relevant gaseous alloying constituent which are lost from the vessel, are registered, an exact assessment of the balance of the quantity of the materials which have been supplied to the metal alloy can be carried out.
The method according to the invention can be used with various electrothermal pressure melting processes where the melt after nitriding is poured into a permanent mould or form. However, the method is used to particular advantage in electrothermal remelting methods where the melting process takes place more or less remotely from the solution equilibrium of the nitrogen between the metal phase and the gas phase.
According to the invention, a device particularly suitable for carrying out the method comprises: a vessel for containing an atmosphere and a metal alloy at overpressure; a heating device by means of which a melt can be generated from the metal alloy; a metering device for feeding into the vessel an alloying constituent which in its normal state is gaseous; a metering device for feeding into the vessel an inert gas; a pressure sensor for registering the total pressure of the atmosphere contained in the vessel; a temperature sensor for registering the temperature of the atmosphere in at least one location; a device for determining the concentrations of the inert gas and the gaseous alloying constituent in the atmosphere contained in the vessel; an evaluation unit which evaluates the total pressure, the temperature of the atmosphere and the concentrations of the inert gas and the gaseous alloying constituent; and a control device which controls the metering of the inert gas and the gaseous alloying constituent depending on the result of the evaluation of the evaluation device.